1. Field of the Invention
This invention is an improved flexible printed wiring board.
2. Description of the Prior Art
Over the past 25 years reinforced resins have been used in an ever-widening range of applications. Such materials are used in construction as a substitute for various structural materials, in electronics as printed wiring board, and in diverse recreational fields. The requirements of each application dictate the use of specific reinforcement materials as well as the use of carefully engineered resins. Reinforcement materials include polyester or glass fibers woven into a fabric or used as a nonwoven mat. The resins are usually organic polymers such as polyesters, polybutadienes or epoxies. Depending upon the particular resin, curing may be effected by means of either conventional heating, or exposure to u.v. light or energetic electrons. The fabrication process is also tailored to the specific product being produced. Such processes include wet-layup techniques, matched metal mold processes, vacuum-bag techniques or the more sophisticated pultrusion process.
A particularly successful application of reinforced materials involves their use in rigid printed wiring board. A specific single step technique for producing copper clad rigid printed wiring board involves the pultrusion process and is disclosed in a commonly assigned application Ser. No. 594,580.
Printed wiring board may have a wide range of mechnical flexibility depending upon the particular application envisioned. In certain applications, thin printed wiring board with a high degree of flexibility is required. For example, the circuitry required in certain telephone handsets is most easily included within the handset by means of a suitable flexible printed wiring board. Such printed wiring board must have sufficient flexibility and mechanical integrity to fit within the handset and to conform to its contours. In addition, it must be sufficiently tough to survive exposure to soldering and various punching, drawing, and power screwing operations encountered during assembly. In general, flexible printed wiring board (FPW) must have both high initiation and propagation tear strength, good dimensional and thermal stability as well as low moisture absorption and good electrical insulation properties.
There are two broad categories into which most flexible printed wiring boards fall--supported wiring board and unsupported wiring board. The supported board, unlike the unsupported board, has some type of reinforcement for added strength. Glass or polyester fibers are the two most common materials used as reinforcement in supported printed wiring boards. Mylar and kapton films are two examples of unsupported printed wiring board.
Attempts to improve the mechanical characteristics of flexible printed wiring board while maintaining its electrical characteristics have met with only limited success. In the unsupported FPW class, mylar is found to melt at the soldering temperatures which must be reached to solder the various circuit components to the board. Kapton has high moisture absorption which has a deleterious effect on dimensional stability. In addition, it displays a propensity to blister on exposure to molten solder. It also has an undesirably low propagation tear strength (i.e., less than 8 gms/mil of thickness), although it has adequate initiation tear strength. FPW reinforced with glass fibers has adequate dimensional and thermal stability, but has low propagation tear strength and also tends to crack on sharp bending. Polyester fiber reinforced boards generally have less thermal stability and higher moisture absorption than fiberglass reinforced FPW but such polyester reinforced boards have higher tear strength. Attempts to use combinations of reinforced materials, in triply or short fiber mat configurations, have met with only limited success.